It is well known that charge injection can change the magnetic, electronic and optical properties of materials. Prior-art methods for changing these material properties by charge injection either (1) involve electrostatic gate-based charge injection across a dielectric (so charge injection is limited by dielectric breakdown), (2) use an electrolyte that contacts the transformed material (thereby limiting device applicability), or (3) use dopant intercalation (thereby limiting applicable materials and providing problematic structural changes). These three methods are called dielectric-based charge injection, non-faradaic electrochemical charge injection, and faradaic electrochemical charge injection, respectively.
Dielectric-based charge injection is used for field effect transistor (FET) devices that are critical for both today's electronic circuits and those proposed for the future. In these FET transistor devices, current is carried through a semiconductor channel between source and drain electrodes. The current through this semiconductor (channel) is controlled by charge injection into the semiconductor channel by application of a voltage between the source electrode and the gate electrode, which is separated from the semiconductor channel by a dielectric. This charge injection is that of an ordinary dielectric capacitor, so the amount of charge injection that can be achieved is limited by the breakdown strength of the dielectric. While enormously useful for submicroscopic electronic devices, this dielectric-based charge injection is unsuitable for macroscopic charge injection in materials having macroscopic external dimensions. X. Xi et al. (Applied Physics Letters 59 3470 (1991)) have demonstrated dielectric-based switching of the superconducting transition temperature (Tc) of films of YBa2Cu3O7-x over a 2 K range. The achieved resistance modulation in the normal state can be as much as 20% in the normal state and 1500% near Tc. Using a similar method of dielectric-based charge injection in the oxygen deficient YBa2Cu3O7-x superconductor, J. Mannhart et al. (Applied Physics Letters 62, 630 (1993)) demonstrated that Tc can be changed by up to 10 K. However, the dielectric-based method of Tc switching used by Xi et al. and by J. Mannhart et al. is not applicable for a macroscopically thick superconducting material. In addition, J. A. Misewich et al. (Science 300, 783-786 (2003)) have used dielectric-based charge injection to make an electrically driven light source from a single nanotube. Also, Y. S. Choi et al. (Diamond and Related Materials 10, 1705-1708 (2001)) have used under-gate dielectric-based charge injection to modulate electron emission for field emission displays, but find disadvantage in this application as a result of field-induced electron beam spreading and restrictions on the anode voltage.
Non-faradaic electrochemical charge injection uses nanostructured materials having very high surface area and is applicable for materials ranging from nanoscale materials to bulk materials. However, unless the material is a metal or metal oxide catalyst, the electrolyte is a ceramic held at high temperatures (P. E. Tsiakaris, et al., Solid State Ionics 152-153, 721-726 (2002)) prior-art technologies teach that this charge injection can only be accomplished by developing and maintaining contact of the electrolyte with regions of the material where charge injection is desired, which for macroscopic nanoporous materials includes internal surfaces. In other words, the prior art teaches that non-faradaic electrochemical charge injection into non-catalytic materials generally requires maintained contact of that material with the electrolyte. This non-faradaic electrochemical charge injection has been used to provide electrochemical electromechanical actuators (artificial muscles, see R. H. Baughman et al., Science 284, 1340 (1999), R. H. Baughman et al., U.S. Pat. No. 6,555,945) and liquid-ion-gated FETs (field-effect transistors, see M. Krüger, Applied Physics Letters 78, 1291-1293 (2001)). However, the maintained contact between the electrode (including both internal and external surfaces) and the electrolyte limits applicability of prior-art methods of non-faradaic electrochemical charge injection. For example, the surrounding electrolyte for the above described liquid-ion-gated FETs limits their applicability for gas state sensing—since a sensed gas must first dissolve in the electrolyte before it can be detected, which decreases both device response rate and sensitivity and limits detection capabilities to gases that can significantly dissolve in the electrolyte. In addition, non-faradaic electrochemical charge injection provides the basis for supercapacitors having much larger charge storage capabilities than ordinary dielectric supercapacitors. In the prior art (K. H. An et al., Adv. Funct. Mater. 11, 387 (2001) and C. Niu et al., Appl. Phys. Lett. 70, 1480 (1997)) these supercapacitors are kept in the charged state as a result of maintained contact between the nanostructured electrodes and the electrolyte. Since the electrolyte provides mechanisms for self-discharge, long term energy storage in such a supercapacitor is not possible. Also, the possibility of charging non-faradaic supercapacitors, removing the electrolyte, and then storing energy in the dry-state supercapacitors has heretofore not been conceived.
Faradaic electrochemical charge injection involves the intercalation of ions into a solid electronically conducting electrode material. This method is limited to the types of materials that can incorporate dopant by a reversible process, preferably at room temperature. For example, elemental metals and metal alloys cannot undergo charge injection by this method. Similarly, this method of charge injection is not useable for non-porous materials having three-dimensional covalent bonding. Also, substantial dopant intercalation fundamentally changes the structure of the material and can introduce gross structural defects. As a consequence, de-doping does not completely return the material to the original state. Nevertheless, the faradaic electrochemical method of charge injection has great value, as indicated by the year 2000 award of a Nobel prize for the discovery that dopant intercalation (either chemically or electrochemically) into semiconducting conjugated polymers can convert these semiconductors into metallic conductors. Faradaic electrochemical doping (for conducting polymers and other materials) is used for both primary and rechargeable batteries (Y. Gofer et al., Applied Physics Letters 71, 1582-1584 (1997)), conducting polymer actuators (R. H. Baughman, Synthetic Metals 78, 339 (1996)), electrochromic displays (W. Lu et al., Synthetic Metals 135-136, 139-140 (2003)), the control of membrane ion permeability (P. Burgmayer and R. W. Murray, J. Phys. Chem. 88, 2515-2521 (1984)), the release of drugs and other biochemically active agents (H. Shinohara et al., Chemistry Letters, 179-182 (1985), L. L. Miller et al., U.S. Pat. No. 4,585,652), and electrochemical light emitting displays (G. Yu et al., Science 270, 1789-1791 (1995)). For such devices, dramatic structural changes are typically associated with dopant intercalation, and these charges are not fully reversed on de-intercalation—which limits cycle life. The required dopant insertion and de-insertion processes (called intercalation and de-intercalation) result in slow device response, short cycle life, hysteresis (leading to low energy conversion efficiencies), and a device response that depends on both rate and device history.
The embodiments of the present invention eliminates key problems of these prior-art technologies, by showing that non-faradiac electrochemical charge injection can be maintained, and even developed, at room temperature for regions of the electrode that are not in direct physical contact with the electrolyte. The present discoveries enable materials and device applications that would not be possible, or would be less advantageous, in the presence of locally contacting electrolyte.